The present invention relates to an apparatus for detecting a position of a light spot in an optical disk, and more particularly to an apparatus for detecting a position of a light spot suitable for a tracking servo control or a track-jumping operation control of an optical memory disk (hereinafter referred to as an OMD).
Hitherto, in the tracking control of an optical disk exclusive for reproduction such as a compact disk, detection as to whether or not a light spot for reading information exists on the axis of the recording track of the optical disk (hereinafter referred to as on-track detection) is conducted generally by using the method called the 3-light-beam method or the push-pull method.
The principle of the 3-light-beam method is as follows. Namely, a preceding light spot and a succeeding light spot are irradiated at the positions preceding and succeeding to a light spot of a main light beam for recording and reading information with the same predetermined spacings in direction of the center line of the recording track and with another predetermined spacings in a direction perpendicular to the center line direction of recording tracks (travelling direction of the light beam) with respect to the light spot of the main light beam, respectively. In this case, the preceding light spot and the succeeding light spot are such that the same spacings are provided in directions opposite to each other with respect to the center line of the recording track in a manner that it is put therebetween. Among rays of reflected light from these light spots, a reflected light beam from the preceding light spot is detected by a first photodetector, a reflected light beam from the succeeding light spot is detected by a second photodetector, and a light spot from the main light beam is detected by a third photodetector. Then, an electric output S.sub.s1 from the first photodetector and an electric output S.sub.s2 from the second photodetector which have been subjected to photoelectric conversion are inputted to the subtraction circuit to perform the following calculation: EQU TE.sub.a =S.sub.s1 -S.sub.s2.
Thus, a differential output TEa is provided. At this time, if the center of the main light beam exists on the axis of the recording track, the following relationship holds: EQU S.sub.s1 =S.sub.s2
Namely, EQU TE.sub.a =0.
In contrast, if the center of the main beam deviates from the center line of the recording track, the difference output TE.sub.a becomes a positive or negative value which is not equal to zero. Accordingly, when a control is conducted so that the value of the difference output TE.sub.a is equal to zero, the tracking servo control can be carried out. Further, counting the points at which the value of TE.sub.a is crossing the zero point (hereinafter reffered to as zero cross points) is equivalent to counting the number of recording tracks. By carrying out this, the positional control of a light spot in the track jumping operation to move over many tracks in a radial direction of the disk can be conducted. Generally, this difference output TE.sub.a is called a tracking error signal. In this case, the output TE.sub.a may have the relationship expressed as TEa=S.sub.s2 -S.sub.s1.
However, it should be noted that the above mentioned on-track detection can be carried out by the 3-light-beam method only in connection with an optical disk exclusive for reproduction. In such an optical disk exclusive for reproduction, there is a small quantity of reflected light at the portion of the recording track because pit trains where information signals are stored are recorded, whereas there is a great quantity of reflected light at the portions except for the recording track because they are subjected to mirror finish. Accordingly, values of TE.sub.a which are not equal to zero are provided.
However, in the case of OMD of the Write Once Read Many (WORM) type or the Erasable Direct Read After Write (E-DRAW) type, at the non-recorded portion where no information is recorded, there is hardly difference between a quantity of reflected light at the portion of the track on which information is to be recorded (which particularly called a groove in the case of the OMD) and that at the portions except for the above. Accordingly, even if an attempt is made to provide a tracking error signal by the above-mentioned calculation, the value of TE.sub.a becomes equal to substantially zero. Thus, both in the on-track state where the center of the reading light spot exists on the center line of the recording track and in the off-track state where the center of the reading light spot deviates from the center line of the recording track, the value of TE.sub.a becomes equal to zero. As a result, the on-track detection cannot be carried out.
Even in such a case, the push-pull method is known as the method permitting the on-track detection. In the push-pull method, a bisected or half-split photodetector is used. By unevenness due to the recording tracks (grooves) on the recording surface of the optical disk and the groove portions except for the above, the zero-th order reflected light and .+-.1-st order reflected light are produced. When these rays of the reflected light are emitted onto the photodetector, the portion having a relatively high light intensity and the portion having a relatively low light intensity are produced. In this case, when an output from one detector surface of the bisected detector surfaces is S.sub.A and an output from the other detector surface is S.sub.B, if a light spot exists on the center line of the recording track (groove), the difference output TE.sub.b =(S.sub.B -S.sub.A) becomes equal to zero. Thus, this method can be used for the on-track detection.
However, even in the case where this push-pull method is used, when the disk is inclined in a radial direction thereof or the optical axis of the lens of the optical system deviates to any degree, an offset is produced. When such an offset is produced, even if the light spot exists on the center line of the recording track (groove), the difference output TE.sub.b does not equal to zero.
As the example of the on-track detection method attempting to solve the above-mentioned offset, the method called 3-push-pull method is known.
This system uses three light beam spots in the same manner as in the case of the 3-light-beam method. In this case, the preceding light spot and the succeeding light spot are both emitted also onto recording tracks TR.sub.i+1 and TR.sub.i-1 adjacent to the both sides of the recording track TR.sub.i on which the main light spot exists. Namely, a portion of a light at the outer periphery of the preceding light spot is emitted also onto the recording track TR.sub.i+1. Also in the case of the succeeding light spot, a portion of a light at the outer periphery thereof is also on the recording track TR.sub.i-1. Reflected light beams from the respective light spots are detected by the three half-split photodetectors. Then, respective difference outputs are calculated on the basis of the push-pull method. When the push-pull difference of the main light spot is TE.sub.M, the push-pull difference output of the preceding light spot is TE.sub.s1, and the push-pull difference output of the succeeding light spot is TE.sub.s2, calculation expressed below is performed: EQU TE.sub.d =TE.sub.M -(K.sub.o /2).times.(TE.sub.s1 +TE.sub.s2).
Thus, an output TE.sub.d can be provided. In the above equation, K.sub.o represents real number of a ratio between a push-pull difference output from the main light spot and a push-pull difference output from the preceding or succeeding light spot in the on-track state. In the on-track state, K.sub.o =TE.sub.M /TE.sub.s1 =TE.sub.M /TE.sub.s2. In the case where the center of the main light spot exists on the center line of the recording track, the output TE.sub.d becomes equal to zero. Accordingly, the tracking control can be carried out also at the non-recorded track portion without undergoing the influence of an offset.
However, there was a problem that in the case of the so called track jumping operation such as an operation to search a desired music piece at a high speed in compact disks, etc., the on-track detection cannot be carried out by the 3-push-pull method. The reason therefor is as follows. In the case of the search operation, the number of zero-cross points is counted to retrieve a target track, but discrimination between the on-track position and the off-track position could not be made only by information about the zero-cross points.
Further, with the conventional 3-push-pull method, also in the tracking control at the time of writing (recording) information onto OMD, etc., there was the problem.
Namely, at the time of writing information, pits are formed by the main light spot, but a quantity of reflected light from the light spot on the pits is small. Accordingly, when a portion of the succeeding light beam is positioned on the pits, the output of the photodetector is lowered. From this fact, even if the main light spot exists on the center line of the tracks (grooves), the tracking error signal TE.sub.d dose not seemingly becomes equal to zero. Thus, judgment might be erroneously made as if deviation were produced in the tracking.